This winter's groundbreaking Leonardo da Vinci exhibition runs for three
months, but took years to put together.

Earlier this week, the National Gallery in London announced a historic collaboration with the Department of Paintings at the Louvre. The French have agreed to lend their version of Leonardo da Vinci’s Virgin of the Rocks to the eagerly awaited Leonardo da Vinci exhibition that opens at Trafalgar Square in November. A few months later, the English will repay the debt by sending Leonardo’s highly finished preparatory drawing The Virgin and Child with Saint Anne and John the Baptist (the Burlington House Cartoon) to Paris, where it will hang in close proximity to the painting it was made for, which is owned by the Louvre.

That might sound like a simple case of high-level back-scratching – except that when the pictures involved are by one of the greatest artists who ever lived there is nothing simple about it. Exhibitions like these don’t just happen. They take years of foresight, planning, perseverance, patience and diplomacy. Leonardo da Vinci: Painter at the Court of Milan offers a textbook example of how shows of this scale and complexity are put together.

More than five years ago, Charles Saumarez Smith, the former director of the National Gallery, asked his curator of Italian paintings before 1500 to come up with ideas for future exhibitions that would illuminate aspects of the collection under his care. At the time, Luke Syson was just beginning to discuss a hugely ambitious and technically difficult project with the gallery’s head of conservation, Larry Keith – the restoration of Leonardo’s Virgin of the Rocks.

As their talks progressed, Syson became immersed in the history of the picture, which the artist painted soon after he arrived in Milan in 1482-83. When it gradually became clear that the painting was going to emerge from Keith’s studio utterly transformed, excitement grew. Syson began to envisage an exhibition that would celebrate the completion of the restoration by focusing on Leonardo’s first Milanese period.

Before he did anything else, Syson had to be clear in his own mind about why he wanted to do such a show, what it might tell us about Leonardo that we didn’t already know, and about when and where similar exhibitions had taken place in the past. If the curator doesn’t ask those questions, his director will. For if potential lenders conclude that the exhibition proposal is pointless or revisits familiar territory, they’ll turn down his requests for loans.

In this case, there were several compelling reasons for lenders to pay close attention to Syson’s idea. First, he was a well regarded scholar who worked for one of the world’s great museums. Next, London has seen several exhibitions devoted to Leonardo, but all had focused on his work as an inventor or as a draughtsman. This show would look at Leonardo the painter, at a stage in his career when he created some of his most ravishing works.

But no institution operates in a vacuum. The very first thing Syson did was to telephone his colleagues at Windsor. The Royal Collection has the most important group of Leonardo drawings in the world. He needed to be sure he could borrow from its holdings in order to provide the exhibition with the proper context within which to show the pictures. When he was told that, in principle, the Queen was willing to lend, his project became viable.

When Nick Penny became director of the National Gallery in 2008, he gave Syson the green light. Penny’s enthusiastic support was vital, because although Syson is a rising star in the study of Renaissance art, he is still young. Penny is old and powerful. His personal intervention in negotiating difficult loans would ensure the success of the show.

In general, the further ahead a museum can plan an exhibition, the more likely its requests for loans will be answered positively. If, as happened in the recent Manet show in Paris, the curator does not plan far enough ahead, many of the most important paintings will be unavailable for loan. One great advantage the National Gallery had was that the show was not going to travel, so the pictures would be subjected to relatively little vibration, and would only be away from home for three months.

Any curator tries to secure the most important loans first, because once he has been promised a painting of world-class importance it becomes easier to convince other owners that this is the show to lend to. I remember when I was doing the Whistler show for the Tate Gallery in the early 1990s, flying out to Cincinnati, Ohio to see his early masterpiece At the Piano. My colleague (from the National Gallery in Washington) had no expectation of success, because the picture was delicate and had never been lent to an exhibition before.

When we arrived at the director’s office, she greeted us with the words, “I’ve been waiting for you for 20 years”. She had turned down loan requests, knowing that one day the definitive Whistler exhibition would take place at some of the world’s great museums. Our show was going to London, Paris and Washington: nothing like it would happen again. She agreed to our request without hesitation.

So Syson started by negotiating the loan of the Lady with an Ermine from the Czartoryski Foundation in Cracow. Next he asked his colleagues at the Louvre for La Belle Ferronnière. With two such stunning portraits secured for the show, it would have been hard for Veneranda Biblioteca Ambrosiana in Milan to turn down his request for Leonardo’s Portrait of a Musician, because with the addition of the two versions of the Virgin of the Rocks it looked like every surviving picture painted by Leonardo in Milan would be in the show.

But why do such institutions agree? What’s in it for them? Building up professional contacts between museums is an important part of the jobs of both the director and the curator. Personal friendship often comes into play.

For example, the Louvre agreed to Syson’s request to borrow not only La Belle Ferronnière but several important drawings. In part that may have been because a few years ago the National Gallery lent Mantegna’s Agony in the Garden for the Louvre’s once-in-a-lifetime Mantegna exhibition – which was a generous act, since the picture is on panel, and therefore fragile. The Louvre had responded by sending Ghirlandaio’s Old Man and his Son to the show at Trafalgar Square about the Renaissance portrait. The director and curators at each institution clearly believed that their pictures would be safe, and that the exhibition and its catalogue would be a serious contribution to scholarship.

And the story didn’t end there. Several curators from the Louvre saw the National Gallery’s Virgin of the Rocks when it was in Larry Keith’s studio. As they watched the picture’s colour and depth emerge from under layers of varnish, they decided that the Louvre should restore one of its greatest Leonardos, The Virgin and Child with Saint Anne. This has now been successfully completed and will be the focus of a show at the Louvre next spring.

Tentatively, Vincent Delieuvin, the Louvre’s curator of paintings, asked Syson whether the National Gallery might consider lending the cartoon or full-scale drawing for the picture. That must have been a difficult moment. The Burlington Cartoon is so fragile that requests to lend it are usually turned down. And yet, after the Louvre’s generosity, it was a request that Syson and his colleagues felt they had to take seriously.

First, Larry Keith said that the work could travel to Paris, taking every precaution to ensure its safety. Scholars at the British Museum and British Library were consulted and they, too, approved the loan.

At this stage, what had been decided was simply that, in principle, the loan could happen. Next the gallery needed to decide whether it should happen. The issue was debated at length by the board of trustees, who decided in the end that if such a loan were ever to happen, this was the moment. And so their precious cartoon will travel to Paris.

'Leonardo da Vinci: Painter at the Court of Milan’ is at the National Gallery, London, from November 9 to February 5, 2012